The refining reaction in a top-blown converter and in a top- and bottom-blown converter proceeds by supplying an oxygen gas from a top-blown lance to oxidize impurities such as carbon, silicon, phosphorus, etc. Furthermore, the top-blown lance usually employs a convergent-divergent nozzle having a single aperture or a plurality of apertures in order to efficiently convert the secondary pressure of the lance into kinetic energy of a jet of oxygen gas, and as a result, the stirring in a steel bath is promoted by the jet. ("Handbook of Steels", 3rd edition, separate volume II, the Japanese Association of Steels, 1982, p. 468).
In order to impart stirring force to a steel bath according to a conventional method, the top-blown lance as described above is used and the refining is carried out under a secondary pressure within a proper range of expansion of the convergent-divergent nozzle from the first period of refining up to the last period of refining, however, an optimum flow rate or a velocity of jet of oxygen gas depending upon the refining steps cannot be selected freely. At the rate determining step of supplying oxygen in the initial period of refining, therefore, when the flow rate of oxygen gas is increased to increase the rate of decarburization, the velocity of jet of oxygen gas is increased, as a result, the amount of dust and spitting increases. At the rate determining step of supplying carbon in the last period of refining, furthermore, when the flow rate of oxygen gas is decreased to prevent super oxidizing of the steel bath and increasing the iron oxide in the slag, the velocity of jet becomes so small that the temperature at a hot spot where jet impinges on the steel bath drops or the stirring force becomes insufficient, resulting in a decrease in the rate of decarburization.
In general, the following three requirements are necessary for the decarburization in the converter, i.e., 1 in a high carbon range, dust is generated less and the slag is formed quickly, 2 in an intermediate carbon range, the decarburization oxygen efficiency is high, and 3 the decarburization proceeds up to a low carbon range while suppressing the formation of iron oxide.
Among them, it has been considered that the converter dust of 1 is generated from two sources, i.e., the dust is generated from a surface (hot spot) where the top-blown oxygen impinges the steel bath, namely, is generated by vaporization of iron from the high-temperature hot spot or is generated by volumetric expansion of a molten steel which occurs when the CO gas is formed by the decarburization reaction at the hot spot.
A variety of methods have heretofore been proposed to increase the iron yield by decreasing the amount of dust generated during the blowing in the converter.
Japanese Unexamined Patent Publication (Kokai) No. 2-156012 discloses a method by which the height of the lance is increased and an inert gas is mixed into the top-blown gas in order to decrease the amount of dust formation. According to this method, the post combustion rate increases accompanying an increase in the height of the lance, and the heat transfer efficiency decreases. Therefore, melt loss increases considerably in the converter refractories. Besides, inert gas is used in large amounts, which is disadvantageous.
According to "Materials and Processes", Vol. 7, 1994, p. 229, the generating rate of dust is dependent upon a value that is obtained by dividing the oxygen supplying rate by the area of hot spot. When the supplying rate of oxygen is lowered to lower the oxygen supplying rate per a unit area of a hot spot, the productivity decreases. When a nozzle having many apertures is used to increase the area of hot spot, on the other hand, the hot spots are overlapped one upon the other causing the splash to increase. When the height of the lance is increased, furthermore, the post combustion rate increases causing the heat transfer efficiency to decrease. Therefore, melt loss occurs conspicuously in the converter refractories.
Japanese Unexamined Patent Publication (Kokai) No. 62-223424 discloses technology for increasing the post combustion rate by using a top-blown lance nozzle that is greatly deformed like that of a star type. Though there has been described no effect of this technology for decreasing dust or splash, simple use of this lance does not help decrease the dust.
When these technologies for lowering dust are summarized, the velocity of jet of the oxygen gas arriving at the bath surface can be decreased, i.e., the jet velocity (u) can be lowered or, in other words, a soft blow is accomplished. In a state of soft blow, however, only a small stirring force is produced by the top-blown gas, and the temperature drops in the region (hot spot) where the jet of oxygen gas impinges the bath surface. Therefore, the decarburization oxygen efficiency starts decreasing from a range of a high carbon concentration, and the above-mentioned object 2 is not fulfilled.
There has further been proposed technology for maintaining a high decarburization efficiency even in the low carbon concentration range 3 mentioned above. For example, Japanese Unexamined Patent Publications (Kokai) Nos. 60-131908 and 60-63307 disclose technology for mixing a top-blown oxygen gas and an inert gas as represented by argon together in the ultra-low carbon range. These methods, however, require argon gas in large amounts, resulting in a great increase in the cost of gas.
In order to fulfill the above-mentioned objects 1 to 3, therefore, it is the best method to supply large amounts of oxygen in a soft blowing manner in the high carbon range, to supply large amounts of oxygen in a hard blowing manner in the intermediate carbon range, and to supply small amounts of oxygen in a hardly blowing manner in the low carbon range.
Japanese Examined Patent Publication (Kokoku) No. 47-4770, on the other hand, discloses a lance provided with c spindle having an operation mechanism that moves up and down in a tubular passage between the opening at an end of a circular oxygen nozzle of the top-blown lance and a throat portion (narrowest portion of the lance nozzle). In this case, oxygen flows through slit portions formed in gaps between the circular nozzle and the spindle, but the jets passing through the gaps meet together immediately after the opening to establish a hard blow. Even when the gaps are broadened, therefore, a soft blow is not realized.
Furthermore, Japanese Unexamined Patent Publication (Kokai) No. 1-123016 discloses a lance having a nozzle for inert gas such as Ar or CO.sub.2 in addition to a nozzle for supplying oxygen. In this case, even when the flow rate of the oxygen gas is lowered, the velocity of the jet doss not decrease due to the inert gas. However, since the oxygen gas is supplied from only one kind of nozzle, the skull is formed on the nozzle to clog it when the flow rate of the oxygen gas is greatly lowered. It is not, therefore, possible to greatly change the flow rate of the oxygen gas or the velocity of jet.
Japanese Unexamined Patent Publication (Kokai) No. 1-219116 discloses a lance having a main hole and a sub-hole which is coupled to an oxygen-supplying pipe which is independent from the main hole. Due to the problem of clogging of the nozzle caused by forming the skull, however, it is not allowed to greatly decrease the flow rate of the oxygen gas. Besides, since the oxygen gas is supplied through both the main hole and the sub-hole, it is not possible to greatly change the flow rate or the velocity of the jet of oxygen gas.